Phosphanilic acid [p-aminophenylphosphonic acid] has been reported in the literature as having antibacterial activity against various microorganisms. It has now been found that mixtures of phosphanilic acid and trimethoprim [2,4-diamino-5-(3',4',5'-trimethoxybenzyl)pyrimidine] in ratios of from about 0.25:1 to about 20:1 exhibit surprisingly high synergistic activity against many strains of bacteria.